1. Field of the Invention
The present invention relates to an ultra-high strength cold rolled steel sheet, specifically to an ultra-high strength cold rolled steel sheet having 75% or higher of hole expansion ratio after blanking, specified by the Standard of Japan Iron and Steel Federation, JFST1001-1996, and having 880 to 1170 MPa of tensile strength, and to a method for manufacturing the same.
2.Description of Related Arts
Responding to the need of reduction in weight of automobiles in recent years, ultra-high strength cold rolled steel sheets having 880 to 1170 MPa of tensile strength are applied to automobile seat frames. Since the automobile seat frames are prepared by press-forming, the ultra-high strength cold rolled steel sheets are requested to have excellent stretch-flangeability, specified by JFST1001-1996, having 75% or higher of hole expansion ratio after blanking.
On the other hand, bumpers and reinforcements for doors conventionally adopt ultra-high strength cold rolled steel sheets having 880 MPa or higher of tensile strength. Aiming at further improvement in their formability and weldability, various studies have been conducted. For example, JP-B-2-1894, (the term xe2x80x9cJP-Bxe2x80x9d referred herein signifies the xe2x80x9cexamined Japanese patent publicationxe2x80x9d), discloses a method for manufacturing an ultra-high strength cold rolled steel sheet having around 1000 MPa of tensile strength, which contains 0.10 to 0.20% C, thus providing excellent cold formability and weldability. JP-B-8-26401 and Japanese Patent No. 2528387 disclose an ultra-high strength cold rolled steel sheet that has 1470 MPa or higher of tensile strength, and excellent formability and impact characteristics by establishing fine martensitic single phase structure or by controlling the volumetric fraction of the martensite in a range of from 80 to 97%. Furthermore, Japanese Patent No.2826058 discloses an ultra-high strength cold rolled steel sheet having 1000 MPa or higher of tensile strength, inducing no hydrogen embrittlement by controlling the martensitic structure and the Fe-C based precipitates.
Those types of conventional ultra-high strength cold rolled steel sheets are, however, often subjected to successive roll-forming because they are used as bumpers and reinforcements of doors, as described above. Accordingly, they were not requested to have excellent stretch-flangeability after blanking. As a result, all of these types of steel sheets have around 50% of hole expansion ratio specified by JFST1001-1996, at the maximum, which level of hole expansion ratio is not applicable to the skeleton members for automobile seat, manufactured by press-forming.
JP-B-5-10418 discloses a high tensile strength steel sheet for laser machining, which has excellent stretch-flangeability. The steel sheet, however, has a low tensile strength of 800 MPa, and the steel sheet is not applicable to the currently used automobile seat frames.
An object of the present invention is to provide an ultra-high strength cold rolled steel sheet having 75% or higher of hole expansion ratio after blanking, specified by JFST1001-1996, and having 880 to 1170 MPa of tensile strength, and to provide a method for manufacturing the same.
The object of the present invention is attained by an ultra-high strength cold rolled steel sheet having 880 to 1170 MPa of tensile strength, which consists essentially of 0.01 to 0.07% C, 0.3% or less Si, 0.1% or less P, 0.01% or less S, 0.01 to 0.1% sol.Al, 0.0050% or less N, and 1.6 to 2.5% of sum of at least one element selected from the group consisting of Mn, Cr, and Mo, and/or 0.0005 to 0.0050% B, by mass, and balance of Fe, and has an inner zone deeper than 10 xcexcm from the surface of the steel sheet being substantially martensitic single phase structure.
That type of ultra-high strength cold rolled steel sheet is manufactured by a method comprising the steps of: producing a steel slab having above-described composition; hot rolling the steel slab into a steel sheet, followed by cold rolling; and heating the steel sheet by continuous annealing method to temperatures of from 800 to 890xc2x0 C., applying primary cooling to the annealed steel sheet at a cooling rate of 20xc2x0 C./sec or less, and applying secondary cooling to the primarily cooled steel sheet at temperatures of from 680 to 750xc2x0 C. to temperatures of 50xc2x0 C. or below at a cooling rate of above 500xc2x0 C./sec.